[BUGFIX beta] Ensure aliased {{with}} blocks are not shared.

Use prototypal inheritance to ensure that the keywords available within
a `{{#with}}` block are not leaked outside the `{{/with}}`.

Added tests to prevent regression.

Closes #3652.
Closes #3753.

(@twokul && @hjdivad && @rjackson)
[DOC beta] Add specs for Ember.EventDispatcher#setup
Remove duplicate changelog entry.
Merge pull request #3769 from pangratz/add/specs_for_event_dispatcher

[DOC beta] Add specs for Ember.EventDispatcher#setup
Merge pull request #3754 from rjackson/use_new_context_for_with_helper

[BUGFIX beta] Ensure aliased {{with}} blocks are not shared.
Merge pull request #3806 from ugisozols/remove_dup_changelog_entry

Remove duplicate changelog entry.